A Doppler coding of the received signals generated by the movement of the carrier vehicle is decisive in connection with processing of raw SAR data. Basically, an SAR processor arranges all targets, whose Doppler frequency appears to be zero (0), in the image in an exactly radial direction (i.e. perpendicularly to the direction of flight). It is therefore basically possible to exactly bring into focus and position non-moving fixed targets.
However, in the course of this, targets which move in a fixed target environment and therefore have a different relative velocity in relation to the aircraft are possibly not displayed or not brought into focus or, shifted from their actual position as function of the different Doppler frequency depending on their radial velocity, they are indicated at wrong places in the image (in this case the tangential shift in respect to the original, actual position is a direct measurement of the radial velocity).
The problem of parameter calculation by means of SAR of moving targets in respect to location, velocity, amplitude, etc., was first investigated by K. Raney in 1971 (see R. K. Raney, Synthetic Aperture Imaging Radar and Moving Targets, IEEE Transactions AES-7, No. 3, May 1971). The primarily optical methods of SAR processing at that time permit only incomplete statements regarding the presence, the exact location and the velocity of moving targets.
However, the observation and measurement of moving targets gains increasing importance today, because the computer technology and therefore also the SAR processing technology has been developed correspondingly. Up to now, a solution to the problem was attempted in accordance with the following four methods:
1. A. Freeman suggests in the article "Simple MIT Using Synthetic Aperture Radar", IGARSS 84 Proc., ESA SP215, pp. 65 to 70, to employ a very high pulse-repetition frequency and a narrow antenna lobe. However, frequency ranges occur in the process in which not fixed targets, but the signals of basically moving targets with corresponding radial velocities, appear. By means of this, moving targets can be detected and their radial velocity determined.
2. It furthermore has been proposed by Klemm and Ender in "New Aspects of Airborne MTI", IEEE Proc. International Radar Conference, Washington, May 1990, to employ an array of four antennas and four receiver candles for receiving SAR signals. It was said to be possible by means of this to determine the velocity vector of moving targets in a clutter environment within a clutter frequency band.
3. K. Ouchi proposes a multilook method, wherein the change in position of moving targets is determined by the change detection in the course of reviewing several images taken in sequence (multilook) (see K. Ouchi, "On the Multilook Images of Moving Targets by SAR", IEEE Transactions AP-33, No. 8, August 1985, pp. 823 to 827).
4. S. Barbarossa and A. Farina suggest the use of a Wigner-Ville distribution for the detection and bringing into focus of moving targets (see S. Barbarossa and A. Farina, "A Novel Procedure for Detecting and Focusing Moving Targets with SAR Based on the Wigner-Ville Distribution", IEEE Proc. International Radar Conference, Washington, May 1990). This is data processing similar to the matched filter method.
The methods proposed by K. Ouchi as well as S. Barbarossa and A. Farina are theoretical suggestions, not yet realizable, for the separation of fixed and moving targets. In both cases a very high signal/clutter amplitude ratio is required. Exact positioning is difficult and the measurement of the velocity vector is extremely inexact.
The method proposed by Klemm and Ender is very promising; however, a very large hardware expenditure in the form of four antennas with four receiving candles is required; this can possibly mean a large weight, a circumstance which is crucial in a negative way, in particular in connection with applications in satellites. Furthermore, blind zones occur in principle, within which a velocity measurement is not possible. More than that, the measuring range basically extends between 3 km/h to 600 km/h.
Only rapid targets can be detected with the method of A. Freeman, and either images of the fixed targets or of the moving targets can be obtained, but not both at the same time. Furthermore, only the measurement of the radial velocity component is possible, the tangential component can not be determined.